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      Java过渡Kotlin08：泛型和枚举
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    <h1 id="Kotlin泛型"><a href="#Kotlin泛型" class="headerlink" title="Kotlin泛型"></a>Kotlin泛型</h1><h2 id="1-类"><a href="#1-类" class="headerlink" title="1.类"></a>1.类</h2><p>声明</p>
<pre><code>class Box&lt;T&gt;(t: T) &#123;
    var value = t
&#125;
</code></pre>
<p>创建</p>
<pre><code>val box: Box&lt;Int&gt; = Box&lt;Int&gt;(1)
// 或者
val box = Box(1) // 编译器会进行类型推断，1 类型 Int，所以编译器知道我们说的是 Box&lt;Int&gt;。
</code></pre>
<p>example</p>
<pre><code>class Box&lt;T&gt;(t : T) &#123;
    var value = t
&#125;

fun main(args: Array&lt;String&gt;) &#123;
    var boxInt = Box&lt;Int&gt;(10)
    var boxString = Box&lt;String&gt;(&quot;Runoob&quot;)

    println(boxInt.value)//10
    println(boxString.value)//Runoob
&#125;
</code></pre>
<h2 id="2-泛型函数"><a href="#2-泛型函数" class="headerlink" title="2.泛型函数"></a>2.泛型函数</h2><p>Kotlin 泛型函数的声明与 Java 相同，类型参数要放在函数名的前面：</p>
<pre><code>fun &lt;T&gt; boxIn(value: T) = Box(value)

// 以下都是合法语句
val box4 = boxIn&lt;Int&gt;(1)
val box5 = boxIn(1)     // 编译器会进行类型推断
</code></pre>
<p>实例</p>
<pre><code>fun main(args: Array&lt;String&gt;) &#123;
    val age = 23
    val name = &quot;runoob&quot;
    val bool = true

    doPrintln(age)    // 整型
    doPrintln(name)   // 字符串
    doPrintln(bool)   // 布尔型
&#125;

fun &lt;T&gt; doPrintln(content: T) &#123;

    when (content) &#123;
        is Int -&gt; println(&quot;整型数字为 $content&quot;)
        is String -&gt; println(&quot;字符串转换为大写：$&#123;content.toUpperCase()&#125;&quot;)
        else -&gt; println(&quot;T 不是整型，也不是字符串&quot;)
    &#125;
&#125;

整型数字为 23
字符串转换为大写：RUNOOB
T 不是整型，也不是字符串
</code></pre>
<h2 id="泛型约束"><a href="#泛型约束" class="headerlink" title="泛型约束"></a>泛型约束</h2><p>我们可以使用泛型约束来设定一个给定参数允许使用的类型。</p>
<p>Kotlin 中使用 : 对泛型的类型上限进行约束。</p>
<p>最常见的约束是上界(upper bound)：</p>
<pre><code>fun &lt;T : Comparable&lt;T&gt;&gt; sort(list: List&lt;T&gt;) &#123;
    // ……
&#125;
Comparable 的子类型可以替代 T。 例如:

sort(listOf(1, 2, 3)) // OK。Int 是 Comparable&lt;Int&gt; 的子类型
sort(listOf(HashMap&lt;Int, String&gt;())) // 错误：HashMap&lt;Int, String&gt; 不是 Comparable&lt;HashMap&lt;Int, String&gt;&gt; 的子类型
</code></pre>
<p>默认的上界是 Any?。</p>
<p>对于多个上界约束条件，可以用 where 子句：</p>
<pre><code>fun &lt;T&gt; copyWhenGreater(list: List&lt;T&gt;, threshold: T): List&lt;String&gt;
    where T : CharSequence,
          T : Comparable&lt;T&gt; &#123;
    return list.filter &#123; it &gt; threshold &#125;.map &#123; it.toString() &#125;
&#125;
</code></pre>
<p>声明处型变<br>声明处的类型变异使用协变注解修饰符：in、out，消费者 in, 生产者 out。</p>
<p>使用 out 使得一个类型参数协变，协变类型参数只能用作输出，可以作为返回值类型但是无法作为入参的类型：</p>
<pre><code>// 定义一个支持协变的类
class Runoob&lt;out A&gt;(val a: A) &#123;
    fun foo(): A &#123;
        return a
    &#125;
&#125;

fun main(args: Array&lt;String&gt;) &#123;
    var strCo: Runoob&lt;String&gt; = Runoob(&quot;a&quot;)
    var anyCo: Runoob&lt;Any&gt; = Runoob&lt;Any&gt;(&quot;b&quot;)
    anyCo = strCo
    println(anyCo.foo())   // 输出 a
&#125;
</code></pre>
<p>in 使得一个类型参数逆变，逆变类型参数只能用作输入，可以作为入参的类型但是无法作为返回值的类型：</p>
<pre><code>// 定义一个支持逆变的类
class Runoob&lt;in A&gt;(a: A) &#123;
    fun foo(a: A) &#123;
    &#125;
&#125;

fun main(args: Array&lt;String&gt;) &#123;
    var strDCo = Runoob(&quot;a&quot;)
    var anyDCo = Runoob&lt;Any&gt;(&quot;b&quot;)
    strDCo = anyDCo
&#125;
</code></pre>
<h2 id="星号投射"><a href="#星号投射" class="headerlink" title="星号投射"></a>星号投射</h2><p>其实就是*代指了所有类型，相当于Any?</p>
<p>例子理解：</p>
<pre><code>class A&lt;T&gt;(val t: T, val t2 : T, val t3 : T)
class Apple(var name : String)
fun main(args: Array&lt;String&gt;) &#123;
    //使用类    
    val a1: A&lt;*&gt; = A(12, &quot;String&quot;, Apple(&quot;苹果&quot;))
    val a2: A&lt;Any?&gt; = A(12, &quot;String&quot;, Apple(&quot;苹果&quot;))   //和a1是一样的
    val apple = a1.t3    //参数类型为Any
    println(apple)
    val apple2 = apple as Apple   //强转成Apple类
    println(apple2.name)
    //使用数组
    val l:ArrayList&lt;*&gt; = arrayListOf(&quot;String&quot;,1,1.2f,Apple(&quot;苹果&quot;))
    for (item in l)&#123;
        println(item)
    &#125;
&#125;
</code></pre>
<h1 id="枚举"><a href="#枚举" class="headerlink" title="枚举"></a>枚举</h1><p>枚举类最基本的用法是实现一个类型安全的枚举。</p>
<p>枚举常量用逗号分隔,每个枚举常量都是一个对象。</p>
<pre><code>enum class Color&#123;
    RED,BLACK,BLUE,GREEN,WHITE
&#125;

enum class Color(val rgb: Int) &#123;
    RED(0xFF0000),
    GREEN(0x00FF00),
    BLUE(0x0000FF)
&#125;
</code></pre>
<p>默认名称为枚举字符名，值从0开始。若需要指定值，则可以使用其构造函数：</p>
<pre><code>enum class Shape(value:Int)&#123;
    ovel(100),
    rectangle(200)
&#125;
</code></pre>
<p>枚举还支持以声明自己的匿名类及相应的方法、以及覆盖基类的方法。如：</p>
<pre><code>enum class ProtocolState &#123;
    WAITING &#123;
        override fun signal() = TALKING
    &#125;,

    TALKING &#123;
        override fun signal() = WAITING
    &#125;;

    abstract fun signal(): ProtocolState
&#125;   
</code></pre>
<h2 id="使用枚举常量"><a href="#使用枚举常量" class="headerlink" title="使用枚举常量"></a>使用枚举常量</h2><p>Kotlin 中的枚举类具有合成方法，允许遍历定义的枚举常量，并通过其名称获取枚举常数。</p>
<pre><code>EnumClass.valueOf(value: String): EnumClass  // 转换指定 name 为枚举值，若未匹配成功，会抛出IllegalArgumentException
EnumClass.values(): Array&lt;EnumClass&gt;        // 以数组的形式，返回枚举值
</code></pre>
<p>获取枚举相关信息：</p>
<pre><code>val name: String //获取枚举名称
val ordinal: Int //获取枚举值在所有枚举数组中定义的顺序
实例
enum class Color&#123;
    RED,BLACK,BLUE,GREEN,WHITE
&#125;

fun main(args: Array&lt;String&gt;) &#123;
    var color:Color=Color.BLUE

    println(Color.values())
    println(Color.valueOf(&quot;RED&quot;))
    println(color.name)
    println(color.ordinal)

&#125;   
</code></pre>

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